The coagulation cascade is initiated by binding of coagulation factor VIIa (FVIIa) to its cell surface receptor, tissue factor (TF). Tissue factor is essential for hemostasis, but the aberrant expression or activation of TF leads to thrombosis, the precipitating event in acute myocardial infarction, unstable angina, and ischemic stroke. It also contributes to inflammation and cancer. Therefore, the proper regulation of TF expression and the activity is critical for not only to maintenance of the hemostatic balance, but also for health in general. Interestingly, the majority of TF on cell surfaces exists in a cryptic state, i.., with no or little coagulant activity, despite forming complex with FVIIa. A stimulus is required fo cryptic TF to become procoagulant active form. A variety of cellular alterations transforms cryptic TF to coagulant active TF. Our recent studies show that 4- hydoxynonenal (HNE), one of the most abundant and bioactive species produced by the lipid peroxidation, activates TF in monocytic and endothelial cells. At present, it is unclear how the coagulant active TF differs from the cryptic form or mechanics involved in TF encryption and decryption. It is unknown, at present, whether phospholipids present in the outer leaflet of plasma membrane play a critical role in maintaining TF in the cryptic state. Of all the proposed mechanisms, externalization of anionic phospholipids at the outer leaflet of plasma membrane following cell activation appears to be the main mechanism for transformation of the cryptic TF into active TF. However, other mechanisms, such as protein disulfide isomerase (PDI)-mediated thiol-disulfide exchange reactions, may also play an important role in TF activation in certain cell types. Mechanisms responsible for maintaining TF in a cryptic state in naive cells and molecular pathways responsible for exposure of anionic phospholipids in response to pathophysiologically relevant stimuli are unknown. The following specific aims are designed to fill these gaps of the knowledge on the regulation of TF activity at the cell surface. Aim 1: Test a novel hypothesis that high sphingomyelin content in the outer leaflet of the plasma membrane is responsible for maintaining TF in its cryptic state at the cell surface. Here, we will also test that the hydrolysi of sphingomyelin in the plasma membrane plays a key role in TF decryption; Aim 2: Elucidate signaling mechanism(s) involved in externalization of phosphatidylserine and TF activation by HNE, and ascertain contribution of PDI-mediated thiol-disulfide exchange pathways and lipid raft integrity in this process. To strengthen observations made in cell systems, and to have better understanding of how cell membrane lipids influence TF activity, we will also perform additional studies with purified TF incorporated into defined liposomes. The data obtained from the proposed studies will provide new insights into understanding of the regulation of TF activity on cell surfaces. Overall, the knowledge gained from the proposed studies will be helpful in understanding the pathogenesis of thrombotic disorders and will be useful in designing better treatment strategies for both thrombotic and hemorrhagic diseases.